Microwave transmission line



Dec. 11, 1956 ARDIT] E AL 2,774,046

MICROWAVE TRANSMISSION LINE Filed May 8, 1952 2 Sheets-Sheet 1 1 6 4 'la /l/l/ll/ha Li 2 5 m L2 INVENTORS MAURICE AfPD/T/ PHILIP PARZEN 1-1 repay/Ev; 9),-

;; Foe (02v; MIND/2 ATTORNEY Dec. 11, 1956' M. ARDlTl ETAL 2,774,046

MICROWAVE TRANSMISSION LINE Filed May 8, 19s: 2 Sheets-Sheet 2 I a, M 2 4 4 INVENTORS MA UEICE A IPD/T/ PHIL /P PARZEN ATTORN EY United States Patent" MICROWAVE TRANSMISSION LINE Maurice Arditi, Clifton, N. J., and Philip Parzen, Forest Hills, N. Y., assignors to International Telephone and Telegraph Corporation, a corporation of Maryland Application May 8, 1952, Serial No. 286,764

14 Claims. (31. 333 s4 This invention relates to microwave-transmission lines and more particularly to a parallel conductor type of line capable of propagating microwave energy in a dominant mode.

In the copending applications of D. D. Greig and H. F. Engelmann, Serial No. 234,503, filed June 30, 1951, now U. S. Patent No. 2,721,312, dated October 18, 1955, a type of microwave transmission line is disclosed comprising in one of its simplest forms two conductors printed or otherwise disposed in substantially parallel relation on opposite sides of a strip or layer of dielectric material a small fraction of a quarter line wavelength thick. While the two conductors may be of the same width it is preferable to have one wider than the other.

One of the objects of this invention is to provide a parallel conductor line of the type above described, wherein the optimum conductor-dielectric relationship is determined to insure wave propagation in a mode simulating essentially the TEM mode.

Another object of the invention is to provide parallel conductor-dielectric type of lines of various conductordielectric arrangements wherein the dielectric relationship with respect to the conductors is such as to minimize the excitation of undesired modes from a wave propagated along the line in a TEM mode.

One of the features of the invention is the arrangement of the dielectric between two ribbon-like conductors whereby the conductors are spaced in substantially parallel relation a small fraction of a quarter line wavelength apart and the lateral extent of the dielectric with respect to the line conductor, that is the narrowest of the two conductors, is maintained such as to minimize the cxsubstantially to an integral number of half Wavelengths.

This lateral extent of the dielectric strip, however, may be limited to an odd number of quarter wavelengths where the lateral edges of the dielectric are coated with conductive material. If magnetic material is used for coating the lateral edges of the dielectric, then it is preferable to extend the dielectric an integral number of half wavelengths beyond the lateral edges of the line conductor. By following this relationship, a line is obtained over which microwave energy may be propagated in the TEM mode with a minimum of insertion loss. The references to wavelengths used herein will best be understood from the following:

(a) The free space wavelength in air M is defined by:

where c is the velocity of light (3x10 cm. per second) and f the frequency in cycles per second.

(b) The free space wavelength in the dielectric Xe where 7m is the free space wavelength in air ands is real part of the complex dielectric constant e (c) The surface wavelength As is the wavelength of a TM type wave parallel to the surface of a plane conductor coated with a thin layer of dielectric, usually for practical Microstrip lines, for instance As-0.99 \o For all practical purposes it can be considered that (d) The wavelength of the principal mode propagating M=khe with kZl k being a function of the dielectric thickness and of the width of the strip conductor (e) The dielectric guide wavelength A for a TM mode excited in a dielectric slab with no ground plane and no strip conductor l n l, \/l For usual Microstrip lines:

)\e 4 and Xgzhe (f) By comparing Equations (11) and (c) it can be seen that:

It will also be readily apparent that the wavelengths he and 7\s may be referred to by employing the free space air wavelength A0 as a common basis with the qualification that it is adjusted according to the quality of the dielectric of the strip. For accuracy of disclosure, however, the following detailed description makes reference to the difierent wavelengths dependent upon the structural arrangement of the conductors and the dielectric strip.

In part (2) above the term n is an integer which expresses the order of the mode, and yo represents the length of the line.

The above-mentioned and other objects and features of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a view in perspective illustrating one form of transmission line in accordance with the principles of this invention;

Fig. 2 is a longitudinal cross-sectional view showing a coaxial junction for launching a microwave for propagation along the line;

Fig. 3 is a graph of losses of the line shown in Fig. 1 when the dielectric strip is varied in width; and

Figs. 4 through 13 show cross-sectional views of a number of transmission lines made in accordance with the principles of this invention.

Referring to Figs. 1 and 2, the microwave transmission line shown is of the printed circuit type comprising a first or line conductor 1 and a second or base con- Patented Dec. .11, 1956 the ductor ZWith a layer 3 of dielectric material therebetween: The conductive material" may'be" appliedand/or shaped or etched on a layer of dielectric material, such as polystyrene, polyethylene, Teflon, fiberglass, quartz, 01' other suitable-materiaLofhigh dielectric quality; in--' the form of strips, conductive paint or ink, or the conductive material may be chemically "deposited, sprayed through a stencil, or dusted onto'seleeted prepared surfaces of the dielectric', or by any other errhe know-n printed circuit techniques. The spacing of the two .tconductors is pref erably selected a small fraction in the order of about to about /5 of a quarter line wavelength of the microwave propagated therealongr It -is known heretofore that with a-transmissionline made of two infinite-plane parallel conductors-a TEM" mode may be propagated .therealong if properly excited. On the other hand, if one of the conductors is omitted,

I the propagation of" a 'TM *type of -wave" parallel to the surface of the dielectric canabe obtained. When a double conductor micrawave line is printed on. alayer of die. electric;irhasbeenfoundby experiment that a dominant mode having the characteristics of a TEM mode may be obtained alongthe center portion .of the line and-that other "modes present-appear to have the. characteristics of'a surface wave propagated along the dielectric and in the dielectric space between the conductors. Experimenthasrshown that there is some coupling between. the TEM mode and the surface wave on thisrtypetoflline. We have found that this coupling canbev greatly minimized 'by properly proportioning the dieletric'strip in width with respect to the Width of the line conductor 1; By making the line with the proper conductor-dielectric relationship, insertion losses can be kept to a minimum, it being obvious that surface modes established through or on the surface of the dielectric produce extra high losses. If the transfer of energy can be prevented from the region I, Fig. 1, from being transferred-tome region II thereof, the losses canbe maintained small. Mathematically, this meansvthat H =Hz= on the dielectric faces." Assuming such to be the'case'and' thatmicrowave energy from source 4, or a coaxial line coupled to the conductors 1, 2 as shown in Fig. 2, is applied to the conductors 1 and 2 for propagation therealong, the modes that exist in the region I may be represented as follows:

(b) Thosemodes which vary with Z. These will be below cut-01f if M; .being 1- the. free sp ace: wavelength. in. dielectric.

(c), Those; modes whichhvary withixronlytn These modes are:

TE modes-r Where 21:1, 2, 3,-etc., ev is the-natural,logarithmicbase, and y isxthe apropagationzconstant'iri thery direction". Such a mode cannot exist since it willgbezbelowit'cnt off for-:sma'll value-of xi. However; if:x12is-.-'sufiiciently large, this mode.- will; propagate-randiexcite :fieldskin region.:II-. T M m0rles.-.These1 modes, also: CZlIJDOlZfiXiStLSiHCCJ, Ey

must,vanisl1 ;forrZ,.=0:'and.,Z=Zo.. Thus the only modes are aaTE-M; modee andtippssibly one. FEE; mode; I Ndwwhat, are: the: possible .-:modes ain regionrlli In view of the: great. mismatchgbetween the: dielectric 4; v and air, it is a good approximation to assume that the component" of magnetic' field parallel to" the" interface between dielectric and air vanishes. conditions in region II are:

and it is desired. to obtain conditionsthat-fon x=0; Hy=Hz'=O;.'

Let us first consider those modes which propagate along the y axis These modes which 'vary-with-"Z'Zare automatically eliminated as .theywillvnotpropagate for Those-modes:- which vary 'withx: 0111mm 1 TE moder woe This mode cannot existsince H must vanisli fo'r TM: modes:

This mode, although it is a propagating-mode willlnot be.excited because it cannot couple to theTEMlmode in region Liwhich possesses only an Themodes. that propagate along the x axis. inregion II. are as follows: As previously these, modes .which vary 1 with Z'Twill. not propagate. The -other. .modes..are:

TE modes:

H-sin wee-[5:

These.are. the:only.modes propagating in; region II.. The guide. Wavelength. Ag. (inlthedielectric) ,of :these.;modes .is given by. Y

Now Hz=0 for x=xn. Hence Hz=0 at x=0 if region I to regipnill hastbeengsatisfiedvbyichoosing Thus the boundary properly the width of the dielectric extending beyond the edge of the conductor 1. Taking n=1 and 7\e 4}'0 this lateral extension is given by This theory becomes more accurate as the dielectric constant of the dielectric increases. If the entire bottom of the dielectric is coated with metal, as in Fig. 7, then the propagating mode along the x axis in region 11 is a surface wave. In this case x should be an integral number of half surface wavelengths of the surface wave if the end faces are uncoated.

Referring more particularly to Fig. 3, the curve therein shown as 9 represents the losses in the line, Fig. l, as a function of the ratio of the dielectric width to Re. It will be observed that the losses were least in the vicinity where x=0 or an integral number of When a plate was added to the line crosswise of the conductor 1 so as to enhance excitement of multimodes through and on the surface of the dielectric, the losses followed substantially the curve 10 which exceeded the losses of curve 9 until the half Wavelength value for x was exceeded and thereafter the losses increased rapidly. This curve 10 is periodic as indicated at 11. This coupling of the TEM mode of the line in Fig. 1 to other modes such as TE and TM was greatest when the value x0 equaled where 11:1, 3, 5, or

na /e 4 where 21:1, 3, This also was substantiated by the curve 9 in the absence of the coupling plate. It is to be understood that the results illustrated by the curves of Fig. 3 represent the losses of a region of an infinite line far. removed from the junction coupling wave energy thereto.

Referring to Figs. 4 through 13, various line constructions are illustrated wherein a minimum of multimode coupling and, therefore, a minimum of line loss may be experienced. In Fig. 4 the two conductors 1 and 2 are separated by dielectric 3a where the value x =0. This form of line is found to give low losses as indicated by the curve 9. It was noted that when the dielectric strip 3a was made narrower than the line conductor the line losses seemed to increase thereby indicating a possible increase of field concentration in the dielectric or excitation of undesired modes. In Fig. 5 the dielectric 3b is shown to have a value such that The losses for this type of line is illustrated for the first dip in the curve 9. Fig. 6 shows another embodiment similar to the form shown in Fig. 4 wherein the base conductor 2a is considerably wider than the conductor 1 or the dielectric 3a. This extent of the conductor 2a decreases slightly the losses experienced in the form shown in Fig. 4. In Fig. 7 the line is shown wherein the di electric 2b extends to an amount such that in Fig. 4. The extent of the dielectric 2b appeared to imnimize the excitation of dielectric wave modes. It was believed that the extended dielectric presents a greater attenuation to the excitation of such modes in this form. The losses experienced in this form of line are illustrated by the curve 12, Fig. 3.

In Fig. 8 the dielectric strip is shown to have about half the extent of that illustrated in Fig. 7, that. is, a value such that By extending the base'conductor 2b around the side edges of the dielectric 30, as indicated at 13, a line characteristic may be obtained substantially as that obtained in the form of Fig. 7. In Fig. 9, a similar relationship is obtained even where conductors 1 and 2 are the same as in Figs. 1, 4, and 5, with the exception that additional layers of conductive material 14 is placed on the lateral side edges of the dielectric 30. It was also found that a similar line eifect could be obtained wherein the dielectric has an extent such that the value if magnetic material is applied to the side edges of the dielectric. Such a line is shown in Fig. 10. There the dielectric 3b is provided with a layer of magnetic material 15 on the lateral side edges, the magnetic material being preferably of very highmagnetic permeability, such as Ferrite.

The double line illustrated in Fig. 11 is similar to the form shown in Fig. 5 except that the space between adjacent conductors 1a and 1b is equal substantially to Likewise, the dielectric 16 thereof is extended laterally of the line conductors on the free sides thereof by an amount corresponding to The base conductors 2 may be identical to the line condnctors 1a and lb or if desired may extend the full width of the dielectric 16. In Fig. 12, for example, the base conductor 17 is shown to extend crosswise of the dielectric 18 and also along the lateral sides thereofas indicated at 19. The space between the line conductors 1a and 1b will now correspond to k, e

and the spacing between the outer edge of the line conductor and the conductive portions 19 corresponds to An extension of the multichannel cable of Fig. 12 is shown in Fig. 13. Two such multichannel strips are formed back to back into a cylindrical cable. The cylindrical cable comprises one such strip 20 coated on one side with a layer of conductive material 21 and between the abutting edges, as indicated at 22. The line conductors are disposed on the other surface of the dielectric strip 20, as indicated at 23 and 24. The spacing between the conductor portion 22 of the base conductor and the next adjacent line conductors corresponds to The spacing between adjacent line conductors, such as 23 and 24, corresponds to much I of; the cylindrical: cablei: could: comprise: a finished; cable-:whereinr. the: coating 21: may. comprise .a copper; braid: coated with dielectric; and rthe 1 usual? protective jacket. It is important, hoiwever,ithat a break becmade'in the dielectric strip soastoprovide-either an air: gap, or :a lateralnconductoni such .as: indicatedi at 22. Such an air gap or groove in the dielectricvstrip provides a desired dielectric-to-air mismatch impedance to minimize multimoding *and th'e possibility of interchannel coupling or crosstalk. Such air gap or conductive partition may be provided in the dielectric strip between" adjacent channels, if, desired; Such provision of airgaps .or, partitions provide additional shielding be tween adjacent channels. The strip20-is accordingly shown provided with such partitions at 22a, 22b, and 220.

In the embodiment shown in Fig. 13, the cable is also shown to comprise a secondilayer of dielectric 25 having a p artition 26. The same copper braid or other conductive coating 21 may serveas the base conductorfor both sections. On the outer surface of the dielectric strip 25 are disposed a plurality of independent line conductors, such' as indicated'at 127' and282' Here again thespacing between the conductive partition 26 and'th'e next adjacent line conductors, such as 27 is in accordance with The spacing between adjacent line conductors, such as 27 and 28, is according to Ifdesired," such a cable may be further coatedfwith' dielectric and asuitable jacket; It is' preferable, however, that the line conductors be provided'with an airspace adjacent thereto to insure low. insertion losses.

While we have described above the principles of our invention in connection with specific apparatus, itis .to be'clearly understood thatthis description'is made only by way of example and not as'alimitationtto'the scope of our invention as set forth in the objects thereof and in the accompanying claims;..

We claim: i

l. A-transmission'cableforpropagation of a plurality oftra-dio'frequency. wave channels in ardominately TEM mode comprising a base conductor of tubular form; a layerof dielectric disposed on one'side of-said tubular conductor, a plurality of line conductors disposed on said dielectric layer, saidzlineconductors being substantially parallel to each other and spaced apart substantially one half a surface Wavelength, and said base conductor having at least one conductive strip disposed radially thereof through said dielectric at a point spaced substantially halfway between two adjacent line conductors.

2; A' transmission cableuaccording to 'claim 1, wherein said dielectric layer is disposed on the inside surface of said tubular conductor. a

3. A transmission cable according to claim 1, wherein the dielectricilayer is dis posed on the outer surface of said tubular conductor.

4. A transmission 1 cableaccording 'to' claim 1, wherein said' base; conductor includes aplurality of conductive strips'l extending through the dielectric, oneeach halfway between adjacent ones of said'line conductors.

5.:- A.transmission cable for propagation of a plurality ofrradio'fr'equency wave channels in a'dominately TEM modencomprising a. base conductor of tubular' form,-a

layer of dielectric? disposed on the insidesurface'of said tubular conductor, a plurality of line conductors disposed on said dielectric layer, said line conductors being substantially parallel to each other and spaced apart substantially one balfcarsurfacez wavelength; 3, vsecond layer of dielectric material disposed ,ontthe outside .surfaceiof said base conductor, and a; plurality of line conductors disposed in substantially parallel relation on the outer:

surface of said second layer of dielectric spaced" apart substantially-one half a-vsurface wavelength, andsaid. base conductor having conductive strips disposed radially one" each through each of the layers of dielectric.

6. A transmission line for'propagation of radio frequency. wave energy in a mode simulating substantiallythe TEM mode, comprising a pair of conductors, a strip. of dielectric material separating said conductors in closely spaced substantially parallel relation,.the mutually opposed surfaces oflsaid conductorslhavinga lateral extent corresponding substantially to the width of atleast one of said conductors and said dielectric strip being of a width greater than saidone conductor such that each of its lateral edges extends beyond the corresponding lateral edge of said one conductor by an amount equal substantially to an integral number of quarter wavelengths oftheffree space air wavelength adjusted according to the quality ofthe dielectric of said strip at which excita} tionof wave modes through or over. the surface of'the" said dielectric strip carries an electrically conductive layer at'the lateral edges thereof and said given fraction of the free space air wavelength is one-quarter.

9. A-transmission line according to claim 6, wherein said dielectric strip carries a layer of ferromagnetic material of high permeability on the lateral'edges thereof;' 10. A transmission line according'to claim 6, wherein said strip is extended laterally and said conductorsare multipled at least on one side of said strip to provide a plurality of channels, the lateral spacing between the conductors of said adjacent channels correspondingsubstantially to a half of one of said adjusted wavelengths or an integral multiple thereof.

11.: A transmissionline according to claim 6, wherein said? given fraction of the free space air wavelength is one-quarter andi-"the width of'the other of said conductors .is equal to substantially thewidth of said dielectric A strip, v sai-d dielectric stripcarrying on the lateral edges thereof a layer of conductive material as continuations of said other conductor. Y I

12'; A transmissionline accordingto claim 11, .wherein said one conductor is multipled to forma plurality of separate line conductors with the spacing betweeniadtjacent lineuconductors equal substantially to a half of one ofsaid adjusted wavelengths or an integral multiple ther'eof'to minimize excitation of wave modes through or-over the surface of the dielectric adjacent each of said line conductors.

13. A transmission line according to claim 12,fwherevinYthe strip of dielectric is of tubular shape ,with *the line conductors on one side of said tubular, strip and said other conductor is on the other side thereof, the conductive material being disposed on the lateral edges of the dielectric strip comprising a conductive partition in the dielectric tubing.

14; A transmission lineaccording to claim v12, further including aconductivepartition. extending fronisaid other conductor through the" dielectric strip su-bstantially half .way between adjacent line. conductors.

References'Cited in the file of this-patent UNITED STATESPATENTSv i 

